Brovyshki/Form1.cs

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace Lyahov_Ekzamen
{
    public partial class Form1 : Form
    {
        Lyahov_EkzamenEntities1  db = new Lyahov_EkzamenEntities1(); 
        public Form1()
        {
            InitializeComponent();
        }

        private void button2_Click(object sender, EventArgs e)
        {
            if (string.IsNullOrEmpty(textBox1.Text) || string.IsNullOrEmpty(textBox2.Text))
            {
                MessageBox.Show("Введите данные для входа");
                return;
            }
            var user = db.lp.AsNoTracking().FirstOrDefault(A => A.login == textBox1.Text && A.Password == textBox2.Text);
            if (user == null)
                MessageBox.Show("Пользователь не найден");
            else
            {
                Form4 f = new Form4();
                f.Show();
                this.Hide();
            }
            
        }

        private void button1_Click(object sender, EventArgs e)
        {
            Form2 f = new Form2();
            f.Show();
            this.Hide();
        }
    }
}